perf: replace concat-with-sourcemaps with custom function

src/utils/concat.ts

@@ -0,0 +1,44 @@
+import { SourceMapGenerator, RawSourceMap } from "source-map";
+import { Extracted } from "../loaders/types";
+import { mm } from "./sourcemap";
+
+interface Concatenated {
+  css: string;
+  map: RawSourceMap;
+}
+
+export default async function (extracted: Extracted[]): Promise<Concatenated> {
+  const sm = new SourceMapGenerator({ file: "" });
+  const content = [];
+  let offset = 0;
+
+  for await (const { css, map } of extracted) {
+    content.push(css);
+
+    const consumer = await mm(map).toConsumer();
+    if (!consumer) continue;
+
+    consumer.eachMapping(m =>
+      sm.addMapping({
+        generated: { line: offset + m.generatedLine, column: m.generatedColumn },
+        original: { line: m.originalLine, column: m.originalColumn },
+        source: m.source,
+        name: m.name,
+      }),
+    );
+
+    if (consumer.sourcesContent) {
+      for (let i = 0; i < consumer.sources.length; i++) {
+        sm.setSourceContent(consumer.sources[i], consumer.sourcesContent[i]);
+      }
+    }
+
+    consumer.destroy();
+    offset += css.split("\n").length;
+  }
+
+  return {
+    css: content.join("\n"),
+    map: sm.toJSON(),
+  };
+}